This invention relates to an electric and hydraulic control circuit for a vertically storing dock leveler, offering, among other features, enhanced control over a platform of the leveler in emergency situations.
There are horizontally storing and vertically storing dock levelers. Horizontally storing dock levelers are stored so that their platforms are raised into position for operation, and vertically storing dock levelers are stored so that their platforms are lowered into position for operation. A conventional horizontally storing dock leveler usually is installed in a pit in a bay of a loading dock. The platform version is raised into position using an hydraulic pump which pumps fluid, usually oil, through various control valves to and from hydraulic cylinders. Once the deck is raised to a sufficient height, and the lip is extended, the deck is allowed to fall by gravity so that it may float up and down with the truck or other vehicle being loaded.
Some horizontally storing models have employed an hydraulic circuit including a normally open (N/O) main solenoid valve, and an electrically operated "emergency stop" valve to hold the deck in a partially raised position. An example of the electrical and hydraulic control circuits for this conventional approach are shown in FIGS. 1 and 2.
In FIG. 2, a pump manifold 1 contains a shuttle valve 25, a pump 30, and a motor 35. When the motor 35 is energized, and the pump 30 is started, fluid is caused to flow from a reservoir 7 through the pump 30. The shuttle valve 25 thus is moved to the left, so that fluid flows through the conduit indicated by arrow 25a, out of a primary port 3 of the pump manifold 1.
Fluid flowing out of the primary port 3 enters a lip cylinder 160 at its retract (rod) end, and also passes through solenoid valve 20 to a main lift cylinder 110. Fluid flowing into both places causes the main lift cylinder 110 to extend, and fluid to flow out of the extend (piston) end of the lip cylinder 160 and into a secondary port 5. A sequence valve 15 is set so that fluid entering the secondary port 5 passes through the valve 15 and into the reservoir 7. As a result, the lip of the leveler retracts as the platform extends.
When the main lift cylinder is fully extended, pressure builds at the primary port 3. As a result, the sequence valve 15 is switched over (in FIG. 2, it is moved to the right against pressure exerted by a spring 16), so that flow may be reversed through the secondary port 5. As a result, the lip cylinder 160 extends, causing the lip of the leveler to extend. Thus, the lip extends after the platform extends.
In the course of operation, when an emergency stop switch 10 (FIG. 1) is pressed, the main solenoid valve 20, which is a spool valve, is energized. The valve 20 normally is open, and permits fluid flow in either of two directions when open. However, energization of the valve 20 causes it to close, prohibiting fluid flow in either direction. As a result, the platform cannot be lowered.
With a normally open valve system, it also happens that when the platform is lowered from a raised position, it cannot be stopped unless the pump 30 is started or the emergency stop switch 10 is depressed. Both of these actions require electrical power. Thus, if there is a power failure, the platform will fall because there is nothing to stop it. The fall is controlled by the rate of fluid flow through the left-hand portion of the shuttle valve.
Vertically storing dock levelers, such as the one shown in FIGS. 3A and 3B, special consideration because when they are stored, people can walk all around them. Thus, it is important that the platform 40 not fall for any reason while it is stored, or while it is being lowered. A system such as that described above for horizontally storing dock levelers, then, would be inappropriate for vertically storing levelers, because in the event of loss of electrical power, the platform naturally would lower in an out of control manner, causing a potential danger to those in its path. Even having an electrically operated emergency stop control, for a system with a normally open valve, would not solve this problem because the platform still could fall in the event of electrical failure. Further, the platform could not be raised (i.e. stored) in the event of power failure.
One type of vertically storing dock leveler control circuit uses a normally closed (N/C) main solenoid valve which requires electrical power in order to open. The operation of such a valve, then, would be the opposite of that for a normally open valve. Examples of the electrical and hydraulic circuits for such a system are shown in FIGS. 4 and 5.
In FIG. 5, a pump manifold 150 includes a shuttle valve 152, a primary port 153, a sequence valve 154, a secondary port 155, a reservoir 157, a pump 162, and a motor 164. A main difference between this hydraulic circuit and that of FIG. 2 is that, when the main solenoid valve 100 is in its normally closed position, fluid cannot flow into or out of the main lift cylinder 110, so that the platform remains locked.
In operation, first a lock/unlock switch 130 (FIG. 4) is turned to the unlock position, energizing the solenoid valve 100, and permitting fluid flow through the valve. This fluid flow allows the platform 40 to float at a speed determined by the rate of fluid flow. In normal operation, this switch 130 remains in the unlock position, to permit the platform to float up and down with the truck which is being loaded or unloaded.
To raise the platform, a raise button 120 is pushed, which causes the platform 40 to rise. When the platform is raised, the raise button is released, and the lock/unlock switch 130 is turned to the lock position, deenergizing the solenoid and closing the valve 100 to keep the platform 40 raised. This type of valve requires two types of action, then, to raise and stop the deck. First, the lock/unlock switch 130 must be turned to the unlock position to open the valve (this is done at the start of operation), and second, the raise button 120 must be pressed to start the motor 35 (after operation is complete, and the platform 40 is to be stored). Similarly, to stop the deck, first the raise button must be released, and second, the lock/unlock switch 130 must be turned to the lock position (closed) as soon as the raise switch 120 is released. If the lock/unlock switch 130 is not in the lock position, the deck will fall again.
An additional deficiency of this type of control valve is that, if the lock/unlock switch 130 inadvertently is left in the unlocked position when there is a power failure, the valve will open as soon as power is restored, allowing the platform 40 to fall, because fluid will be able to flow out of the main lift cylinder 110, resulting in a potentially dangerous situation.